1. Field of the Invention
The invention relates to a hydraulic pressure control apparatus for a vehicular power transmitting device provided with a fluid power transmitting device with a lockup clutch through which hydraulic fluid discharged from an oil pump is circulated after being pressure regulated by a relief type pressure regulator valve. More particularly, the invention relates to technology that increases the flowrate when some excess hydraulic fluid that flows out from the pressure regulator valve flows out to the upstream side of the oil pump.
2. Description of the Related Art
A hydraulic pressure control apparatus for a power transmitting device of a vehicle which is provided with a fluid power transmitting device with a lockup clutch through which hydraulic fluid discharged from an oil pump is circulated after being pressure regulated by a relief type pressure regulator valve is well known. This hydraulic pressure control apparatus switches the circulation path of hydraulic fluid depending on whether the lockup clutch is applied or released such that when the lockup clutch is released, hydraulic fluid flows out from the fluid power transmitting device to a first fluid passage that is connected to a cooler on the downstream side. Meanwhile, some excess hydraulic fluid that flows out from a pressure regulator valve flows out to a second fluid passage, while the rest flows out to the upstream side of the oil pump.
Japanese Patent Application Publication No. JP-A-2003-120797, for example, describes one such hydraulic pressure control apparatus for an automatic transmission (i.e., power transmitting device). The hydraulic pressure control apparatus described in Japanese Patent Application Publication No. JP-A-2003-120797 circulates hydraulic fluid that has been pressure regulated by a secondary regulator valve through a torque converter with a lockup clutch. When the lockup clutch is released (i.e., off), the hydraulic fluid flows out of the torque converter and into a fluid passage to which a cooler and a safety valve are connected in parallel on the downstream side. Hydraulic fluid that was heated by agitation from the torque converter is cooled in the cooler. Also, in the technology described in Japanese Patent Application Publication No. JP-A-2003-120797, some of the excess hydraulic fluid that flows out from the secondary regulator valve flows out a fluid passage for lubrication, while the rest flows to the upstream side of the oil pump.
However, in the hydraulic pressure control circuit such as that described in Japanese Patent Application Publication No. JP-A-2003-120797, of the amount of hydraulic fluid supplied to the secondary regulator valve, the amount of hydraulic fluid that needs to be cooled (i.e., the amount that needs to flow through the cooler) has to flow to the cooler. Further, the amount of hydraulic fluid needed for lubrication has to flow to a lubrication passage which is in a different system (i.e., along a different path) than the cooler. Moreover, when there is an excessive amount of fluid circulating through the torque converter while the lockup clutch is released (i.e., off), an excessive amount of hydraulic fluid is discharged from the safety valve, thus increasing waste. As a result, less hydraulic fluid may end up flowing out from the secondary regulator valve to the upstream side of the oil pump, which may be detrimental with respect to cavitation of the oil pump (i.e., cavitation tends to occur more easily), and may make fluid vibration more apt to occur when operating at high temperatures and high speeds, and during extremely low temperature starts, for example, which may lead to abnormal noises and reduced durability.